stl::multimap - how do i get groups of data?

Multimap essentially has groups of data sorted by the key. I want a method by which I could access these individual groups and get their aggregate values.
For example, in a

std::multimap< string, int >

I store

{"Group1", 1},
{"Group1", 2},
{"Group1", 3},

{"Group2", 10},
{"Group2", 11},
{"Group2", 12}

Having stored these values, I should be able to iterate this multimap and get the aggregate values of each "group". Problem is there aren't any functions defined in STL to access MultiMaps in such a way. I could use

lower_bound

,

upper_bound

to manually iterate the multimap and total the group's contents, but I am hoping there could be better ways already defined in STL ? Can anyone propose a solution as to how I could get the aggregate values for a group in the above example.